In recent years, management of data stored in an information recording area of an information recording medium such as a semiconductor memory, magnetic disk, optical disk and magneto optical disc has been realized by a file system. In the file system, the information recording area is divided into sectors as minimum access units and clusters which are aggregations of sectors to manage data. One or more clusters are managed as a file.
A FAT file system is an example of file systems conventionally used. The details of the FAT file system are disclosed in ISO/IEC9293, “Information Technology-Volume and file structure of disk cartridges for information”, 1994.
The FAT file system is a file system generally used for information devices such as personal computers. In the FAT file system, physical location of a lot of data constituting a file is managed under a unified manner using a table referred to as a FAT (File Allocation Table). Since the information recording medium in which data is managed by the FAT file system can share a file between devices interpreting the same file system, data transfer between devices becomes possible.
In the FAT, one fixed-length entry is used for a cluster. Thus, as the capacity of the information recording medium is larger and the number of the clusters managed in the file system is increased, the size of the FAT becomes larger. When installing the FAT file system in a device, a built-in type device having the limited capacity of memory adopts a method of holding only a part of the FAT in the memory and caching the FAT rather than holding the whole of the FAT in the memory to reduce the memory consumption.
Conventionally, there has been proposed a method of inputting information including the number of data managed in the FAT and location of the FAT into an information recording reproduction device from a host device at initialization and caching all or a part of the FAT, as a method of cashing the FAT. This method is disclosed in, for example, Japanese Unexamined Patent Publication No. 8-110868. According to this method, information concerning an accessed file of the information on the FAT is cached on a cache buffer. In this case, fast access become possible, since there is no need to read the FAT on the recording medium in a reaccess to the file accessed once.
However, the above-mentioned conventional technique has the following problem. The above-mentioned data area managing method intends to speed up an access to an existing file and does not consider a free area retrieval processing. According to the above-mentioned data area managing method, although reaccess to the file accessed once can be performed at high speed, when retrieving a free area and preparing a new file, the FAT needs to be newly read from the information recording medium because the FAT is not cached.
To perform the free area retrieval processing in a conventional method, the status of use of each entry stored in the FAT is checked and a cluster number in a free area is obtained. Especially when the number of free area is decreased, the number of entries to be checked in the free area retrieval processing is increased and in the worst case, the whole FAT needs to be read into a cache buffer when retrieving the free area. Here, when a read unit of the cache buffer is small, the free area retrieval processing takes longer time due to the overhead of read processing.
In the light of the above-mentioned problem, the present invention intends to realize a data area managing method that can lessen the overhead at accessing to the FAT by changing the read unit of the FAT according to processings performed using the FAT, for example, the free area retrieval processing and a link destination acquisition processing and to provide an information processor using the data area managing method.